Most of the world's fisheries are threatened by unsustainable fishing practices and pressures. Many fisheries target the adult life phase of the target species as the commercial resource. This is economically effective as long as fishing systems and pressure do not jeopardise the biological integrity of the wild stocks of the target species. However, with many species of marine organism, the individuals that live long enough to become adults numerically represent only a very small percentage of the reproductive effort of the wild population. This small percentage is a result of high mortality rates which occur during the early stages of growth development of the species.
The growth development of many crustaceans and fin fish include five main stages: the egg, larvae, puerulus larvae, juvenile and adult. The eggs are carried by the female until they hatch, becoming larvae. The larvae often drifts in the water freely for about 12 months before becoming puerulus larvae (also known as pueruli or post-larvae). The post-larvae, which look like transparent miniature adults, settle on the bottom of the ocean floor in a phase known as “the settlement phase”. It is on the ocean floor where the post-larvae begin the benthic phase, meaning “bottom of the water”, and they will grow into juveniles and finally full adults.
For many commercially important crustaceans and fin fish, the critical development stages that succumb to the greatest mortality rates are the settlement phase and the succeeding benthic phase. Settlement phase mortality is generally acknowledged to be in the region of 80-95% of settling post-larvae. Regardless of how many post-larvae settle, various studies show that, of the survivors of the settlement phase, very few will survive the subsequent benthic phase to become adults. Mortality in the first year, post-settlement and during the juvenile phase, has been shown to be similarly high. Therefore, the ideal manner in which to harvest commercial crustaceans and fin fish is to target the progeny prior to the settlement phase, as the removal of up to 80% of settling post-larvae, before they encounter these mortality phases will have virtually no effect on the numbers surviving to become adults and negligible effect on related ecology.
These post-larval marine organisms can be harvested and grown out in aquaculture operations, providing the potential for an extremely lucrative and non-destructive fishery for coastal communities throughout the world. Numerous studies have used different collection techniques to harvest post-larvae with light traps, used to harvest photophyllic fin fish, or settlement/collector habitats, used to harvest post-larval crustaceans.
Several different styles and designs of collectors have been used for small scale studies. Most of these designs are variations of the “Witham collector”, which are generally made of closely spaced sheets of air conditioner filter material suspended vertically to form a float, (Witham et al. 1968). Other examples of collectors include the Phillips collector, and variants such as the GuSi collectors (Phillips and Booth, 1994; Phillips 1995), the Mills collector, sandwich collectors and the like. Almost all previous designs of Witham collector described in literature to date, have used natural fibres that have been treated with a rubberising agent to prolong their useful life. More advanced and recent designs have used synthetic fibre material attached to wooden or Styrofoam bases. Because of their cost of manufacture, materials used and/or general design, past designs of post-larval collectors have not been successful for commercial use; being either too costly, not sturdy enough or impractical for large-scale use.
Prior collector designs have encountered issues with ‘sea-worthiness’, unable to withstand harsh sea conditions for long periods of time. Sturdier constructions have proven too costly for commercial use. Additionally, prior designs have not satisfactorily addressed the issues of attracting and retaining post-larvae stage organisms and also result in lower yields as organisms are lost during harvesting.
Thus, there exists a need for, and growing interest in, a device which can aid post-larval stage harvesting on a commercial scale.
The invention described herein addresses these issues; thus providing a cost effective, sturdy design and construction that attracts and retains pre-settlement, post-larval organisms, while also minimizing loss during harvesting.